The present invention relates to a process for producing yellow iron oxide pigments from iron(II) chloride and an alkaline component by the precipitation process, wherein after addition of the xcex1-FeOOH nucleus to the preprecipitated FeCl2 the pH is 3 to 7.
The production of yellow iron oxide pigments by the precipitation process has been known for a long time. The typical course of this process is described, for example, in Ullmann""s Encyclopedia of Industrial Chemistry, 5th Ed., Vol. A20, p. 297 ff. The raw material generally used is iron(II) sulfate obtained during the pickling of steel sheets, or iron(II) sulfate formed during the production of titanium dioxide by the sulfate process. Large quantities of FeCl2 are also obtained in the manufacture of synthetic rutile for the production of TiO2.
However, an increased trend towards the use of hydrochloric acid as a mordant has been observable for years in the pickling industry. Due to its purity, the iron(II) chloride obtained here is particularly suitable for the production of yellow iron oxide. Furthermore, the so-called chloride process is being increasingly used worldwide for the production of titanium dioxide. Hence solutions containing iron(II) chloride, which as far as possible should be converted into a useful material, are increasingly being obtained from both sources. A common process for this conversion is the spray-roasting process, in which the iron(II) chloride or an iron(III) chloride at elevated temperatures (typically more than 1000xc2x0 C.) is subjected to an oxidative hydrolysis. The end products formed here are iron oxide, typically haematite, and hydrochloric acid which, being the required useful material, is reintroduced into the pickling process. Without particular purification steps, the iron oxide obtained is suitable for the production of hard ferrites.
If soft ferrites are to be produced from such iron oxides, it is necessary to subject the iron chloride solution to additional purification operations, which makes the process significantly more expensive. As more and more iron chloride, for the most part of inferior quality, is becoming available, and the absorption capacity of the ferrite market is limited, an alternative process for the production of a useful material from this iron chloride is sought. A direct deposition or dumping of the iron chloride solutions is unacceptable for environmental reasons.
Accordingly, an object of the present invention was the conversion of iron chloride into a high-grade useful material.
In general, xcex1-FeOOH (yellow iron oxide) is produced from iron(II) salts by the precipitation process (DE-A 2 455 158) or by the Penniman process (U.S. Pat. Nos. 1,368,748, 1,327,061). In both processes a nucleus is first of all produced, onto which, in a further step, additional xcex1-FeOOH is allowed to grow relatively slowly. Unlike FeSO4, FeCl2 is usually not isolated as crystalline material, because its solubility in water is significantly higher than that of FeSO4. It is therefore in many cases more highly contaminated than FeSO4, which is in fact purified by the crystallization step. In addition, the FeCl2 solution frequently contains organic constituents, which can greatly influence the crystallization process.
As iron(II) chloride solutions from steel pickling and from TiO2 production frequently contain interfering quantities of cations of higher valency (for example, Ti, Cr, Al, V, Si), these have to be removed by precipitation using an alkaline component (EP-A 0 911 370). The resulting iron chloride has a pH of between 2 and 4.
It has been found that many iron(II) chloride solutions obtained from steel pickling do not lead to pure needle-shaped xcex1-FeOOH when the so-called acid nucleation process is employed (see also EP-A 0 406 633, Example 1). Where FeCl2 is used, instead of xcex1-FeOOH, in many cases xcex2-FeOOH in the form of very long, thin needles is obtained. During the subsequent pigment formation xcex2-FeOOH partially decomposes to form xcex1-Fe2O3, depending on the applied temperature (Chambaere, D. G. and De Grave, E., Phys. Chem. Minerals, 12, (1985), 176-184). Consequently, it not possible to use xcex2-FeOOH as nucleus material for the production of xcex1-FeOOH pigments.
In order for the FeCl2 from steel pickling to be usable, the nucleus has to be produced by the xe2x80x9calkali processxe2x80x9d (U.S. Pat. No. 2,558,304). If one now proceeds with an xe2x80x9calkalinexe2x80x9d yellow nucleus in accordance with the known process for pigment formation, where preprecipitated FeCl2 is used, the addition of the nucleus to the iron(II) chloride produces a pH of 5-7, which at temperatures of above 50xc2x0 C. leads to unwanted black magnetite.
Accordingly, it is an object of the present invention was to find a process whereby yellow iron oxide pigment can be produced from preprecipitated FeCl2 and from a nucleus produced by the alkali process.
1. The invention relates to a process for producing a yellow iron oxide pigments comprising the steps of
I) mixing preprecipitated FeCl2 and an xcex1-FeOOH nucleus produced by the xe2x80x9calkalixe2x80x9d process, wherein
a. the Fe concentration of the FeCl2 is between 70 and 220 g/l,
b. the Fe(III) content of the FeCl2 is less than 8 mol % Fe(III),
c. the pH of the suspension after addition of the xcex1-FeOOH nucleus (measured at 30xc2x0 C.) is 3 to 7,
II) oxidizing the suspension in a first oxidation step, wherein
d. the temperature is 20 to 45xc2x0 C.,
e. the rate of oxidation in the first oxidation step is 0.5 to 10 mol % Fe(III) formed per hour,
f. the pH (measured in suspension at 30xc2x0 C.) at which the first oxidation step is ends is between 1.5 and 3.0, and
III) oxidizing the suspension in a second oxidation step, wherein
g. the temperature is 55 to 85xc2x0 C.,
h. the pH is increased at a rate of 0.05 to 1.0 pH/h by continuous addition of an alkaline component,
i. the end point of the rise in the pH in the second oxidation step is pH 2.4to 5.2,
j. the rate of oxidation in the second oxidation step is 0.2 to 10 mol % Fe(III) formed per hour,
k. the rate of circulation of the suspension during the reaction, based on the final volume of the batch, is between 1 and 15 times per hour.
The invention preferably relates to a process where the following parameters can be adjusted:
a. the Fe concentration of the iron component used is between 85 and 130 g/l,
b. the Fe(III) content of the Fe component used is 0.1 to 2.5 mol % Fe(III),
c. the pH of the suspension after addition of the alkaline yellow nucleus (measured at 30xc2x0 C.) is 4 to 6,
d. the temperature in the first oxidation step is 30 to 40xc2x0 C.,
e. the rate of oxidation in the first oxidation step is 4 to 7 mol % Fe(III) formed per hour,
f. the pH (measured in suspension at 30xc2x0 C.) at which the first oxidation step is carried out is between 2.4 and 2.8,
g. the temperature in the second oxidation step is 60 to 75xc2x0 C.,
h. the pH in the second oxidation step is increased at a rate of 0.1 to 0.8 pH/h, by continuous addition of an alkaline component,
i. the end point of the rise in the pH in the second oxidation step is pH 2.8 to 4.2,
j. the rate of oxidation in the second oxidation step is 0.4 to 4 mol % Fe(III) formed per hour,
k. the rate of circulation of the suspension during the reaction, based on the final volume of the batch, is 1 and 3 times per hour.
The process according to the invention can contain the following steps:
Preprecipitation (not necessary if the FeCl2 used is of high purity)
Sodium hydroxide solution is added, with intensive stirring, to an iron(II) chloride solution with an FeCl2 content of between 50 and 450 g/l, in a quantity sufficient to establish a pH of 3 to 5. Instead of sodium hydroxide solution, other alkaline components can be used including Ca(OH)2, Na2CO3 or ammonia. Moreover, the sedimentation behavior of the accumulating hydroxide slurry or carbonate slurry can be improved by the addition of a flocculation aid. Suitable flocculation aids include the known polyacrylates or other similarly acting substances. Sedimentation behavior may optionally be improved by a subsequent oxidation, resulting in the conversion of various metal cations into oxides or oxyhydroxides of higher valency, which are more easily separable.
The hydroxide slurry can be removed by sedimentation, filtration or separation using a separator, the selection of the suitable apparatus or of the suitable method depending on the exact experimental conditions, the flow rates and the raw materials used.
Production of Nuclei by the Alkali Process
To produce an xcex1-FeOOH nucleus by the alkali process, it is advantageous to use batch-operated stirred-tank reactors, continuous-flow stirred-tank reactors, emitter reactors (without stirrer), loop reactors or tubular reactors.
An Fe component with a concentration of 21 to 150 g/l Fe, preferably 44 to 132 g/l Fe, is added with thorough mixing, over a period of 15 to 95 minutes, preferably 30 to 55 minutes, to a solution of an alkaline component (typically sodium hydroxide solution, sodium carbonate, Ca(OH)2 et cetera). The concentration of the alkaline component is 3-15 equivalents per liter (based on the stoichiometry of the reaction for the nucleus formation). The quantity of alkaline precipitating agent is between 120 and 350% of the stoichiometry; preferably 150 to 250% of the required stoichiometry. The precipitation is carried out at temperatures between 30xc2x0 C. and 60xc2x0 C., preferably 34xc2x0 C. to 47xc2x0 C.
The subsequent oxidation takes place at a rate of 5 to 50 mol % Fe(III)/h, preferably 15 to 30 mol % Fe(III)/h. When all the Fe(II) has been oxidized to Fe(III) (xcex1-FeOOH), the nucleus obtained, after examination (particle size), is used for pigment production without further isolation.
Pigment Formation
Batch-operated stirred-tank reactors, reactors with emitters (two-component nozzles, liquid, gas) loop reactors or bubble columns are particularly suitable for pigment formation. The alkaline nucleus is pumped over a period of 10 to 120 minutes into a solution of an Fe(II) component with an Fe content of 70 to 220 g/l Fe, preferably 85 to 130 g/l Fe. Depending on the ratio of Fe in the nucleus to Fe in the Fe(II) component, excess alkaline component in the nucleus and pH of the Fe component used, a pH of between 3 and 7 (measured at 30xc2x0 C.) is established.
Preferably preprecipitated FeCl2 having a pH of 2 to 4 is used. At the end of the pumping process, the mixture is heated to the first oxidation temperature of 20 to 45xc2x0 C., preferably 30 to 40xc2x0 C. Oxidation is then carried out using an oxidizing agent at a rate of 0.5 to 10 mol %/h Fe(III) formed, preferably 4 to 7 mol %/h. This step is continued until all the Fe(OH)2 formed by the mixing together of the alkaline yellow nucleus and the Fe component has been oxidized to xcex1-FeOOH, which is the case at a pH of 1.5 to 3.0. This is followed by heating to a temperature of 55xc2x0 C. to 85xc2x0 C., preferably 60xc2x0 C. to 75xc2x0 C.
When this temperature has been attained, the pH is increased at a rate of 0.05 to 1.0 pH/h, preferably 0.1 to 0.8 pH/h, by means of continuous addition of an alkaline precipitating agent. At the same time, an oxidizing agent is added continuously at a speed such that the rate of oxidation is between 0.2 and 10 mol %, preferably between 0.4 and 4 mol %, of Fe(III) formed per hour.
The pH is raised no further than pH 2.4 to 5.2, preferably pH 2.8 to 4.2, and the pH is then maintained at this value until the end of the reaction. In order to avoid sedimentation the rate of circulation throughout the entire reaction is between 1 and 15 times the final volume of the reaction.
Depending on the reaction conditions chosen, light or dark yellow pigments are obtained. The most important controlling parameters are shown in the following Table:
Oxidizing agents which can be used, include:
atmospheric oxygen
pure oxygen
ozone
H2O2 
sodium hypochlorite or bleach liquor or calcium hypochlorite
chlorates(III) or chlorates(V)
perchlorates
nitrates
chlorine
The oxidation is concluded as soon as the Fe(II) content of the suspension is less than 1 mol %. A further oxidation to 100% conversion is of less use economically, but can definitely be achieved.
Suitable alkaline components include:
alkali metal hydroxides
alkali metal carbonates
alkaline-earth metal hydroxides
alkaline-earth metal oxides
alkaline-earth metal carbonates
ammonia (as solution or gas)
organic N bases (in special cases)
Sparingly soluble compounds (for example, MgCO3) are used in the form of aqueous slurries. Combinations such that sparingly soluble secondary products would be formed during the reaction should not be used.
Stoichiometry of the Reaction
The nucleus production and the pigment formation take place according to the following stoichiometry:
2 FeCl2+4 NaOH+xc2xdO2xe2x86x922 FeOOH+4 NaCl+H2O
or
2 FeCl2+2 Na2CO3+xc2xdO2+H2Oxe2x86x922 FeOOH+4 NaCl+2 CO2
If insufficient suitable iron(II) chloride is available, then, both during the nucleus production and during the pigment production, the iron(II) chloride may be partially replaced by iron(II) sulfate or by another economical source of Fe. But the preferred embodiment is the exclusive use of iron(II) chloride.
It is also possible to reduce iron(III) chloride obtained during the production of TiO2 by the chloride process to iron(II) chloride by means of metallic iron. This iron(II) chloride solution can then be subjected to the purification by preprecipitation described above.
The yellow iron oxide pigment obtained is particularly suitable for coloring plastics or paper, or for the production of emulsion paints or other coatings and paints.
By suitably varying the specified production conditions, the person skilled in the art will be able to create a broad palette of yellow iron oxide pigments of various particle sizes and hence of various tones.
The pigment can be isolated from the pigment suspension by the known steps of filtration, drying and grinding.
The determination of the properties given in the Examples is carried out as stated in EP-A 0 911 370 (corresponding to U.S. Pat. No. 6,042,642 hereby incorporated by reference).
The invention is described below by means of Examples, but these are in no way intended to be limiting.